1. Field of the Invention
The present invention relates to an exposing method, and more particularly to a projection exposing method in the technology for forming circuit patterns for example of a semiconductor device, or a liquid crystal device.
2. Related Background Art
In the conventional exposing method, the light beam illuminating a reticle pattern enters the reticle with a specified angular range spreading range of light beam) having the center at the perpendicularly incident ray. However said angular range is about 0.6 times of the numerical aperature at the reticle side of the prejection optical system, and does not contain a very large incident angle.
In such apparatus, the resolving power and the depth of focus are mostly determined by the numerical aperture (NA) of the projection optical system and the exposing wavelength (.lambda.). In general the resolving power is represented by k..lambda./NA, and the depth of focus is represented by .lambda./NA.sup.2, wherein k is a constant determined by the performance of the photosensitive material (photoresist), and is in the order of 0.6. Also in the currently available apparatus, NA is abut 0.5, and .lambda. is mainly 0,365 .mu.m (i line of mercury lamp).
Consequently, for obtaining a high resolving power, there can be employed an exposing apparatus in which NA is increased and the wavelength of the exposing light from the light source is shortened. The depth of focus is however drastically reduced in such case, and the mass production of integrated circuits becomes difficult due to deficient focus margin. Also the reduction in the wave-length of the exposing light is difficult to realize due to the lack of suitable optical material or photoresist material.
Recently, new technologies called annular illumination or modified light source are attracting attention. These technologies are to improve the resolving power and the depth of focus of the projection optical system by increasing the incident angle of the illuminating light (namely inclining the illuminating light) to the reticle pattern. These new illumination technologies (hereinafter collectively called modified light source technology) are effective for a line-and-space pattern (grating pattern) , and particularly effective for a pattern in which the opaque portions are comparable to or finer than the transparent portions.
Such modified light source technologies were explained in "New Imaging Technique for 64M-DRAM" in the Optical/Laser Microlithography V, Vol. 1674, pp 741-752, issued at the International Society for Optical Engineering (SPIE) held 11-13 March, 1992.
Also for increasing the depth of focus, there are known methods of moving or vibrating the object of exposure (for example wafer) in the axial direction of the projection optical system during the exposing operation, or effecting exposures in plural positions different in the direction of height of photoresist (these methods being collectively called focus expanding methods). Such methods are particularly effective for an isolated island-shaped transparent pattern present in an opaque background, but such methods alone cannot provide an increase in the depth of focus in so-called line-and-space pattern in which the opaque portions and the transparent portions are regularly repeated. Such focus expanding method is disclosed for example in the U.S. Pat. No. 4,992,825.
Also insolubilization of the surface of a photo-resist film, for example with alkali rinsing, prior to exposure prevents the film decrease of the remaining pattern of the photoresist even for a low-contrast optical image of the reticle pattern, resulting from defocus or deficient resolving power, thereby providing a photoresist pattern with sufficient etching resistance. Such insolubilizing technology is disclosed in the Japanese Patent Laid-open Application No. 63-316429.
However, such surface-insolubilized photoresist tends to assume an inversely tapered profile (top portion of resist pattern being wider than bottom portion; such inversely tapered profile being particularly pronouced in a defocus state in which the wafer is positioned farther from the projection lens), based on an abrupt change in the intensity distribution of the optical image in the vicinity of the wafer surface in a defocus state, whereby the dimension after etching shows significant fluctuation. Consequently such insolubilizing technology cannot be easily applied to a pattern with a strict dimensional precision.
FIGS. 9 and 10(a) to 10(f) show photoresist profiles obtained by exposures in which the wafer surface is positioned at the best focus plane and the focus state is fixed (wafer is fixed). In such case the intensity distribution of the image varies strongly with the amount of defocus .DELTA.F. Corresponding to such variation, the resist profiles at the upper portion (1) , intermediate portion (2) and lower portion (3) of a step on the wafer respectively assume forms shown in FIGS. 10(a), 10(b) and 10(c) or 10(d), 10(e) and 10(f), wherein the profiles shown in FIGS. 10(a), 10(b) and 10(c) are obtained with ordinary photoresist while those shown in FIGS. 10(d), 10(e) and 10(f) are obtained with surface-insolubilized photoresist. Thus, when the intensity distribution of the image varies strongly with the focus state as shown in FIG. 9, there is encountered an overhung profile as shown in FIG. 10(e) so that the surface insolubilization is not too much effective.
Based on the use of a positive-working photoresist, the recently proposed modified light source technologies are effective for a line-and-space pattern in which the opaque portions are comparable to or narrower than the transparent portions, but do not provide significant improvement in the resolving power and in the depth of focus, in a line-and-space pattern in which the opaque portions are wider than the transparent portions.
With the shift of the integrated circuits toward the finer geometry, there is an increased requirement for such line-and-space pattern in which the opaque portions are wider than the transparent portions, namely the resist film portions remaining after positive photoresist patterning are wider than the removed resist portions. Particularly in the dynamic random access memory (DRAM) of stacked capacitor type, in order to sufficiently secure the capacitance per a memory element, it is necessary to reduce the width of the removed resist portion constituting an insulating area between the electrodes of the capacitor, while maintaining the width of the remaining resist portions constituting the electrodes of the capacitor. Since such pattern requires the start of dissolution of positive photoresist at a higher light amount level, the improvement in the resolving power and in the depth of focus by the modified light source technologies is limited, in comparison with a pattern with a duty ratio of 1:1.
The line-and-space pattern with wider opaque portions based on the positive-working photoresist can be transformed into a line-and-space pattern with narrower opaque portions by the use of a negative-working photo-resist, so that the modified light technologies become effective, but this is imptactical because the performance of such negative-working photoresist is currently inferior to that of the positive-working photoresist.
On the other hand, since the stacked capacitors constitute a densely arranged periodical pattern, the conventional focus extending method alone cannot provide an increased depth of focus, due to a fact that such method alone significantly deteriorates the image contrast for such densely arranged periodical pattern.
Also the dimensional fluctuation and the deformation of the pattern of the stacked capacitors lead to a functional error of the device. For this reason, the conventional insolubilization of the resist surface results in a significant pattern deformation due to the above-mentioned inversely tapered profile and is unacceptable in the periodical pattern with strict dimensional precision, such as the pattern of the stacked capacitors.